The invention relates to a spark plug with a full-length longitudinal bore which is designed to hold an insulating element and is positioned eccentrically in the shell, the shell having regions of varying wall thicknesses and a pressure measuring device being incorporated in a region of greater wall thickness.
A quantity essential for the assessment of the combustion process in an internal combustion engine is the pressure prevailing in the combustion chamber. Research and development, engine management and engine control systems require a means for measuring pressure in the combustion chamber. Usually it is desirable to obtain pressure measurements without the necessity of an additional bore in the combustion chamber. One possibility of achieving this aim is to adapt the spark plug for an additional pressure measuring function.
The known devices of this kind may essentially be classified in two groups. A first group uses so-called force measuring disks, which are used instead of a sealing washer or are mounted at the site of the seal of the spark plug and measure the pressure acting on the plug as a change in sealing force. A disadvantage of such a system lies in the fact that the plug shell and the threaded bore holding the spark plug are subject to different deformations due to temperature differences during the operation of the engine, which deformations will immediately act on the force measuring element and will cause significant measurement errors.
The second group uses pressure measuring elements which are contained in the shell of the spark plug, problems in this case being due to the restricted space available for the pressure measuring elements in this area.
In order to obtain more space for the pressure measuring element pressure measuring spark plugs have been developed, whose longitudinal bore for the insulator has been positioned eccentrically in the spark plug shell, thus creating a region of lesser and one of greater wall thickness, the pressure measuring element being placed in the latter.
Multiple variants of insulating elements that are placed asymmetrically in the spark plug shell have been described in EP 0 441 157 A. If the ignition cable or coil connector is to be centrally fitted, a cranked adapter part is required in these instances. Due to the change in the position of the high voltage connector considerable costs for the adapter are incurred, especially in the case of ignition coils, which singly or conjointly are placed directly above the spark plugsxe2x80x94a design occurring with increasing frequency. Furthermore, for a given diameter of the spark plug shell, the insulator must be of a more slender design, which will increase the probability of insulator breakage when such pressure measuring spark plugs are screwed into the cylinder head. Transverse forces exerted by the screwing tool used may be particularly detrimental. The insulator may also be damaged by the fitting forces arising when the plug connector is fitted or when the coil is mounted. Another source of damage to the insulator, which may result in insulator breakage, is due to the inertial forces exerted by plug connector plus cable, which are caused by accelerations during operation of the engine.
A spark plug of the above type is presented in DE 35 14 597 A1. With this spark plug the longitudinal bore is positioned in the shell eccentrically. This will result in a part of the shell with greater wall thickness, in which a pressure sensor is positioned in a stepped bore normal to the axis of the spark plug, the pressure sensor being directly connected with the combustion chamber. Via a sack bore parallel with the axis of the spark plug, which opens into a bore running at an angle thereto in the shell, the electrical signal leads of the pressure sensor are carried to an external evaluation unit. In a second variant a stepped bore is formed parallel with the spark plug axis in the thicker-walled part of the shell, which bore contains a pressure sensor and a sleeve-shaped pressure fitting. With this configuration the pressure sensing device can only be dismounted after removal of the insulating body.
From AT 402 116 B a spark plug is known which includes a force measuring element for determination of the pressure acting on the spark plug. If pressure is applied in axial direction the insulating body of the spark plug is resting against the spark plug shell via the force measuring element. In a special variant the insulating body is divided to reduce the mass coupled to the force measuring element, i.e., into a part on the side of the combustion chamber, which is supported by the force measuring element, and a part on the side of the ignition cable, which is directly attached to the shell. With the use of an electrically conductive intermediate piece the two parts of the inner electrode on the side of the combustion chamber and on the side of the cable, respectively, will also be mechanically decoupled to a large extent. An insulating sleeve is provided between the two parts of the insulating body.
It is an object of this invention to further develop a spark plug including a pressure measuring device of the above type such that the fitting dimensions of a conventional spark plug are obtained while insulator breakage is avoided.
According to the invention this object is achieved by providing that the spark plug insulator be configured as a multi-piece element and comprise at least one upper piece carrying the high voltage connector and one lower piece adjacent to the combustion chamber, and that an insulating intermediate element of flexible material be placed between the upper and the lower piece of the insulator.
By configuring the insulator in two pieces and placing an insulating intermediate element of flexible material between the upper and lower insulator piece, harmful material stresses due to mounting or acceleration forces may be avoided in the slender lower part of the insulator, thus more or less eliminating the danger of breakage. Fitting and accelerating forces may be taken up by the comparatively thick-walled upper piece of the insulator, as the invention provides that at least the high voltage connector and the part of the upper insulator piece facing the high voltage connector should be radially symmetric to the spark plug axis and should have fitting dimensions essentially corresponding to the respective parts of a conventional spark plug. A pressure-measuring spark plug according to the state of the art, with an asymmetrically positioned insulating body, will exhibit significantly lesser wall thicknesses in its upper region, in addition to requiring a cranked adapter piece for fitting of the ignition cable connector or the ignition coil.
The intermediate element is preferably made from a high-temperature resistant elastomer, such as a silicone rubber or a fluoroelastomer. The upper piece of the insulator may be made from plastic or, like the lower piece, from ceramic material.
Since the upper piece of the insulator has fitting dimensions identical with that of a conventional spark plug, no additional cost is incurred for adapting the ignition cable or coil connection. This is of particular advantage in the case of coils which singly or conjointly are placed directly above the spark plugs.
Since, according to the invention, the lower piece of the insulator is protected inside the spark plug shell, a very slender design may be chosen, which will leave ample room for a pressure measuring device in the region of greater wall thickness of the spark plug shell, next to the lower insulator piece.
It is of particular advantage to provide a lateral bore for the pressure measuring device, which is situated in the region of greater wall thickness next to lower piece of the insulator and forms an angle of 0xc2x0 to 15xc2x0, and preferably 4xc2x0 to 12xc2x0, with the spark plug axis. A slight inclination of the bore will facilitate mounting and dismounting of the pressure measuring device.
In a first embodiment of the invention the pressure measuring device is configured as a probe held in the lateral bore. In a second embodiment it is provided that the pressure measuring device comprise a pressure measuring element seated in a recess of the shell adjacent to the combustion chamber and a signal lead guided in a lateral bore.
According to the invention the pressure measuring element may be configured in a known manner as a piezoelectric element subject to pressure by a sensor membrane, the central axis of the sensor membrane preferably running essentially normal to the axis of the lateral bore. Although other orientations of the sensor membrane would be possible, i.e., directly towards the combustion chamber, the orientation of the membrane axis normal to the axis of the lateral bore will permit a relatively large diameter to be selected for the sensor membrane, thus improving measurement.
One of the advantages of the design with an integrated measuring element over that with a probe is that the slender signal lead and the flat measuring element will permit the use of a smaller thread for the pressure-measuring spark plug. In the variant featuring a probe the lower limit of the spark plug thread is reached with the thread dimension M 14 due to the 4 mm diameter of the probe. Probes of greater slenderness are not desirable from a measuring point of view and are difficult to manufacture.
Finally, it is an advantage according to the invention if the axis of the lower piece of the insulator is inclined towards the spark plug axis, such that the tip of the spark plug is brought close to the spark plug axis. Due to the asymmetrical arrangement of the longitudinal bore in the spark plug shell the center electrode of the spark plug usually will not coincide with the spark plug axis. This off-position may be reduced by inclining the axis of the lower insulator piece relative to the spark plug axis.